Membranes are used, for instance, in separation processes as selective barriers that allow certain chemical species to pass, i.e., the permeate, while retaining other chemical species, i.e., the retentate. Membranes are used in many applications, for example as biosensors, heparinized surfaces, facilitated transport membranes utilizing crown ethers and other carriers, targeted drug delivery systems including membrane-bound antigens, catalyst-containing membranes, treated surfaces, sharpened resolution chromatographic packing materials, narrow band optical absorbers, and in various water treatments which involve removal of a solute or contaminant, for example, dialysis, electrodialysis, microfiltration, ultrafiltration, reverse osmosis, nanofiltration and in electrolysis and in fuel cells and batteries.
There are many materials or substrates for membranes. Specific physical and chemical characteristics to be considered when selecting a substrate include: porosity, surface area, permeability, solvent resistance, chemical stability, hydrophilicity, flexibility and mechanical integrity. Other characteristics may be important in certain applications.
The rate of transport through a membrane is inversely related to the thickness of the membrane. However, reduction in thickness is normally accompanied by a loss of mechanical strength. Loeb and Sourirajan prepared a membrane which contained a thin dense layer at one surface and a less dense, more open structure through the rest of the membrane. The thin, dense layer provides the separating function of the membrane while the more porous component assists in providing mechanical strength. Loeb and Sourirajan formed the skinned membranes using a casting, partial evaporation and subsequent phase inversion. These and comparable membranes typically are made from a single polymer using some kind of phase separation process in the production of the film.
A further important type of membrane involves the formation of a thin layer on the surface of a supporting membrane in which the thin film has a different composition to that of the support. Such membranes are generally known as thin film composite membranes. Typically, the thin dense layers consist of a crosslinked polyamide that is made using an interfacial polymerization step.
Essentially all commercially available membranes used in reverse osmosis and nanofiltration applications are of a thin film construct with most being based on the thin-film composite approach.
The present invention is concerned with pore-filled membrances, not thin film membranes. It should be stressed that there is a fundamental difference in membrane construction between thin film and pore-filled membranes. The separating or active layer in thin film membranes is typically a dense layer that is formed on the top of a support membrane. This dense layer faces the feed solution. In pore-filled membranes a low density, cross-linked gel is contained within the pores of a microporous substrate and serves as the separating “layer”. Because of the nature of the gel it has to be prevented from excessive swelling in contact with water or other gel-swelling solvents by the physical constraint imposed upon it by the microporous host. As a result, in pore-filed membranes attempts are made to avoid having gel surface layers but instead to ensure that the gel is held within the pores of the host.
In Mika et al., J. Membr. Sci., 108 (1995) pp 37 to 56, there is described a procedure for modifying microporous polypropylene and polyethylene membranes wherein 4-vinylpyridine is in-situ graft-polymerized into the pores of the membrane. The teaching of this article is incorporated by reference.
U.S. Pat. No. 6,258,276, the disclosure of which is incorporated by reference, teaches that by cross-linking the membranes described by Mika et al. with a suitable cross-linking agent, such as divinylbenzene (DVB), there are provided charged membranes comprising porous microfiltration substrate membranes whose pores have anchored therein a cross-linked polyelectrolyte or hydrogel which exhibit novel effects in a variety of membrane applications. In particular, the membranes are said to exhibit significant ion rejection properties, enabling water softening to be effected, particularly at ultra-low pressure, such as the pressure of tap water, by removing multivalent ions, such as calcium and magnesium, in preference to monovalent ions, such as sodium. The membranes further exhibit electrochemical separator properties which make them suitable for a wide variety of applications, including electrodialysis, battery separators, fuel cell separators and electrochemical synthesis. In addition, the membrane may be used for Donnan dialysis, diffusion dialysis and pervaporation.
Table 1 shows performance data of some symmetric pore-filled membranes at a driving pressure of 100 kPa with a municipal tap water feed, and one commercially available thin-film or non-pore filled membrane (DESAL-51). In each case the pore-filled membranes were made using a poly(propylene) microporous host membrane.
TABLE 1Mass-Cross-FLUX atREJECTIONINCORPORATEDgainlinking100 kPa(%)GEL(%)(%)kg/m2hNaMgCaPoly(vinylbenzyl42512357980ammonium)/PiperazinePoly(vinylbenzyl5397247862ammonium)/DABCOPoly(vinylpyridine)/α,66118207461α′-dichloro-p-xyleneDESAL-518217154
DESAL-51 is a commercially available high performance, flat-sheet nanofiltration membrane produced by Osmonics. The data shown for DESAL-51 in Table 1 were obtained under identical conditions to the pore-filled membranes.
In these pore-filled membranes transport of matter such as solvent or solutes occurs only through the incorporated gel phase and not through the microporous support material. The microporous support simply provides mechanical support for the incorporated gel.
These polyelectrolyte gel-filled membranes have a more or less even distribution of the incorporated polyelectrolyte gel-throughout the thickness of the membrane. This means that the thickness of the active layer, i.e., the thickness of the polyelectrolyte gel layer, is approximately the same as the thickness of the starting host membrane, typically in the range 80 to 120 μm.
Plasma induced graft polymerization techniques, which are well known as a surface modification method, could, in principle, be used to prepare asymmetrically filled porous membranes. Yamaguchi et al. [J. Polym. Sci. Part A. Polymer Chem. 34, 1203-1208 (1996), Macromolecules 24, 5522-5527, (1991) and Ind. Eng. Chem. Res. 32, 848-853 (1993).] have described the plasma graft polymerization of poly(methylacrylate) onto a microporous high density poly(ethylene) membrane, pore size 0.02 μm. It was found that the grafted polymer was distributed symmetrically through the membrane cross-section. The graft polymerization rate was, however, affected by changing the monomer diffusivity relative to the reactivity of activated sites with change of the solvent. This in principle could lead to control of the grafted polymer location in the substrate. The present inventors have found that it is very hard to control the location of the grafted polymer using plasma activation techniques. In particular, with the large pore-sized and high porosity substrates preferred for high performance pore filled nanofiltration membranes experience in introducing poly(acrylic acid) has been that grafting largely occurs throughout the thickness of the membrane. The grafted polymers introduced by this method were not crosslinked.
Porous hollow fibre membranes filled with hydrogels having mesh size asymmetry are disclosed by Dai and Barbari [J. Membrane Science, 171, 79-86, (2000)]. The hollow fibre was first impregnated with a solution of poly(vinyl(alcohol) and gluteraldehyde as a cross-linker so as to form an incorporated gel evenly distributed across the thickness of the membrane. After the cross-linking reaction was completed in the pores, the pore-filling hydrogel was modified to create mesh size asymmetry in the gel phase within the wall of the fibre. A gradient cross-linking was used to create the asymmetry in cross-linking. While there was an asymmetry in cross-linking density through the wall of the fibres the overall gel density did not significantly alter.